TECHNICAL FIELD

The present application relates to an apparatus and method for fracturing a well. More particularly, it relates to a downhole tool and method of operating a downhole tool for fracturing a well.

BACKGROUND

Conventional oil and gas extraction from oil wells has been cost-effective and popular within the oil and gas industry since the late nineteenth century. Conventional oil and gas extraction utilized a rig to drill straight down into an oil and gas pool to obtain high yield of oil and gas and considerable profits have been achieved. However, with more oil and gas w ^? ells being exploited and the diversified permeability of layers varying from low to high, a demand for unconventional oil and gas production has risen. The unconventional oil and gas, unlike conventional oil and gas, is produced or extracted using techniques other than the conventional methods used in prior wells. Formations like shale rock are formed in layered deposits with iow ^? permeability where oil and gas within these layers is spread out in small quantity. To produce from such dispersed oil and gas reserves within these layers, horizontal drilling is employed and methods such as hydraulic fracturing are widely practiced.

Oil and gas production from rock formation involves horizontal drilling and fracturing operations, wherein a fracturing tool such as a fracturing sleeve and a seat is deployed. The fracturing tool is then plugged by utilizing an obstructive component such as a ball onto a seat of the fracturing tool. Pressurized fluid is then employed and diverted by the plugged fracturing tool into the targeted formation. Further, multiple fractures at different depths along the horizontal or vertical well string increases total production through said well string, wherein fracturing tools are positioned at intervals along the well. Similar methodology can be applied, wherein multi-stage fracturing operations employ an obstructive component, a ball for example, to plug the furthest fracturing tool on the well string from the surface control system. When fracturing of the furthest tool is completed, the next furthest tool is then plugged, and the operations of fracturing are repeated until all the stages are successfully tracked.

In such operations, the seat and the ball may obstruct the wellbore in order to divert the pressurized fluid into the wellbore and then at a later time collect and transport dispersed oil back to the surface through the wellbore. Current technology employs self- removing or mechanical methods to clear away the obstruction from the flow' path through which oil and gas is produced. Self-removing methods represent significant cost savings by eliminating certain rig operations including drilling or milling, which present challenges due to the horizontal nature of the well string. Further, quick removal of the obstructing members allows early production and return on capital. Recent developments have introduced self-dissolving halls for such purposes. A self-dissolving seat, on the other hand, involves resolving the challenge to control appropriate timing and speed of seat removal. At present, technicians as well as workers employ methods like drilling to remove the seat and parts of the well string equipment to unblock the flow' path. However, drilling may be costly and time-consuming, which in turn raise the demand for more efficient means of removing the seat or other obstructing members of the well string equipment.

Current technology for dissolvable parts used in hydraulic fracturing involves no activation point. Under this circumstance, the dissolvable part may start to dissolve as soon as it is in contact with the fluid present in the well environment. The ball seals onto the ball seat due to a relatively small interference in diameter, as the ball and seat sizes are designed to maximize the number of zones possible within a single well bore, with each successive ball and seat combination being smaller than the earlier combination by a value close to the interference in diameter. As such, in a relatively short time, the dissolvable ball will not be able to hold pressure as the ball outer diameter is constantly reducing. The commonly used solution is to use a selected material to fit a particular well bore condition and estimated operational time requirement for the ball which will be dropped prior to the fracturing of each zone. Further, parts of the fracturing tool embedded as part of the well siring including the seat and other obstructing members are as yet unable to utilize dissolvable material technology due to long period of contact with well fluid before fracturing operations. Dissolvable materials that are able to withstand the downhole fluid and environment without significant detriment to pressure retaining and other capabilities for the substantial time required until the last of the fracturing operations can be concluded, will require long time period for removal by dissolution and hence obstruct the well bore from production for a long period of time.

SUMMARY

The downhole tool as is described below solves the major problem of removing the ball, seat, sleeve and/or any downhole member in a controlled manner. It also allows standardized dissolvable equipment to be used across various well environments. Further, embodiments of the present invention provide a trigger mechanism for tools at a distance based on certain pre-selected conditions. An activation point for dissolving is introduced and various methods including pressure and shear activation will be illustrated in the following text.

A downhole tool comprises a housing, a seat movably disposed in the housing. The housing has a bore formed therein and a plurality of flow ports in fluid communication with the bore. The seat has a main body, a protective layer covering a surface of the main body to isolate the main body from fluid communication with the bore, and an access portion connected to the main body. The seat is movable relative to the housing from a first position at which the access portion is isolated from fluid communication with the bore, toward a second position at which the access portion is exposed to establish fluid communication with the bore. Upon the access portion being exposed, a fracturing fluid injected into the downhole tool will be in contact with the access portion to dissolve the seat through the access portion. As such, removal of the seat is achieved in a controlled manner, i.e. after the fracturing fluid is injected into the downhole tool.

Other aspects and advantages of the present application will become apparent from the following detailed description, illustrating by way of example the inventive concept and technical solution of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present application are disclosed hereinafter with reference to the drawings, in which:

Fig. 1A is a half-sectional side view of a downhole tool according to a first embodiment, before a fracturing fluid is injected into the downhole tool.

Fig. IB is a half-sectional side view showing the downhole tool of Fig. 1 A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 1C is an enlarged sectional side view' of a seat assembly of the downhole tool of Fig. 1A.

Fig. 2A is an enlarged partial view of portion 12 of Fig. 1 A.

Fig. 2B is an enlarged partial view of portion 14 of Fig. IB.

Fig. 3A is a half-sectional side view of a downhole tool according to a second embodiment, before a fracturing fluid is injected into the downhole tool.

Fig. 3B is a half-sectional side view of the downhole tool shown in Fig. 3A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 4 A is an enlarged partial view of portion 32 of Fig. 3 A.

Fig. 4B is an enlarged partial view of porti on 34 of Fig. 3 B.

Fig. 5A is a half-sectional side view of a downhole tool according to a third embodiment, before a fracturing fluid is injected into the downhole tool. Fig. 5B is a half-sectional side view of the downhole tool shown in Fig. 5A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 6 A is an enlarged partial view of portion 52 of Fig. 5 A.

Fig. 6B is an enlarged partial view of portion 54 of Fig. 5B.

Fig. 7A is a half-sectional side view of a downhole tool, according to a fourth embodiment, before a fracturing fluid is injected into the downhole tool.

Fig. 7B is a half-sectional side view of the downhole tool shown in Fig 7 A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 7C is an enlarged partial view of portion 72 of Fig 7A.

Fig. 7D is an enlarged partial view of portion 74 of Fig. 7B.

Fig. 8A is a half-sectional side view of a downhole tool according to a fifth embodiment, before a fracturing fluid is injected into the downhole tool.

Fig. 8B is a half-sectional side view of the downhole tool shown in Fig. 8A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 8C is an enlarged partial view of portion 82 of Fig. 8 A.

Fig. 8D is an enlarged parti al view' of portion 84 of Fig. 8B.

Fig. 9 is a flow chart showing a method of operating a downhole tool according to various embodiments. Fig. 10 A is a front view of a downhole tool according to a sixth embodiment.

Fig. 10B is a partial exploded view of Fig. 10A.

Fig. 11 A is a sectional side view of the downhole tool shown in Fig. 10A.

Fig. 1 IB is a sectional side view of a downhole tool shown in Fig. 1 1 A, after the downhole tool is set and secured to a well bore.

Fig. 12A is a front view of a downhole tool according to a seventh embodiment

Fig. 12B is a partial sectional side view of the downhole tool shown in Fig. 12A, after a fracturing fluid is injected into the downhole tool and a fracturing operation is performed.

Fig. 12C is a partial secti onal side view of the downhole tool shown in Fig. 12 A, after seat 1218 is dissolved.

Fig. 12D is a partial sectional side view of the downhole tool shown in Fig. 12A, after a ball expands and passes through a bushing.

Fig. 13A is a sectional side view of a downhole tool according to an eighth embodiment, before a dissolvable fluid is injected into the downhole tool.

Fig. 13B is a sectional side view of a downhole tool shown in Fig. 13 A, after the downhole tool is set.

Fig. 13C is a sectional side view ⁷ of a downhole tool of section 13B-13B of Fig.

13B.

Fig. 13D is a sectional side view of a downhole tool shown in Fig. 13 A, after locking elements are dissolved. Fig. 13E is a sectional side view of a downhole tool shown in Fig 13A, after the downhole tool is released.

Fig 14A is a sectional side view of a plug member for use in a downhole tool according to a ninth embodiment, before a rupture disk is burst.

Fig. 14B is a secti onal side view of the plug member shown in Fig. 14 A, after the rupture disk is burst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The foliowing disclosure describes several embodiments for implementing various features, structures, and/or functions of the present invention. Embodiments of components, arrangements, and configurations are described below to illustrate the present disclosure. However, embodiments provided are merely examples and are not exclusive to other possible alterations and/or modifications. These embodiments provided are not intended to limit the scope of the application. Additionally, the present disclosure may repeat reference characters (e.g., numerals) and/or letters in the various embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and may not necessarily in itself dictate a relationship between the various embodiments and/or configurations discussed in the Figures. The formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Moreover, the embodiments presented hereinafter may be combined in any combination of ways, e.g., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure.

Embodiments of the present disclosure provide a downhole tool that has a dissolvable component. The dissolvable component may be a seat, a mandrel, a sliding sleeve, a bridge plug, a plug element such as a ball, a dart, or other component of the downhole tool, which is used during fracturing process, and is desired to be removed after completion of the fracturing to allow flow of the oil/gas from the well bore up to the surface. By way of examples, a dissolvable seat assembly, a dissolvable ball and a dissolvable locking member will be described for use in a downhole tool for fracking operation. The dissolvable seat assembly may also be used in other downhole tools as well. In some embodiments, the dissolvable component includes a main body made of material dissolvable to the production fluid used in the fraction operation, a protective layer covering the main body, and an access portion connected to the main body. The protective payer may be a surface-treated, outer layer of the seat assembly, or a coating applied to the seat assembly. The protective layer may be a non-permeable and a non·· dissolving layer to the production fluid, by which, the protective layer substantially prevents the main body from dissolving, until it is desired to do so by activating the dissolving process by exposing the access portion to the production fluid.

Referring now to the specific embodiments, Figs. 1A, IB, 1C, 2A and 2B show a downhole tool 100 and parts thereof, according to a first embodiment of the present invention. The downhole tool 100 includes an upper sub 136, a lower sub 134 coaxially coupled to the upper sub 136, a housing 140 coupled to the upper sub 136 and the lower sub 134, and a seat assembly 106 slidably disposed in a bore 132 of the housing 140.

The bore 132 forms a flow path through which fracturing fluids maybe injected into the housing 140 and pass through a plurality of flow ports 110 formed in the housing 140 and in fluid communication with the bore 132, for fracturing operation. The upper sub 136 may have a top thread 102, the lower sub 134 may have a bottom thread 104, both for connecting the downhole tool to other tubings or tools (not shown) of a fracturing system for multistage fracturing operation.

A positioning element such as a pin or a shear screw 112 is engaged to housing 140 and seat assembly 106, to fix the seat assembly 106 to the housing 140. The seat assembly 106 includes a seat 108, a shifting sl eeve 109, a main body 105 made of a first material (herein after“dissolvable material”) dissolvable to a production fluid used in fracturing operation, a protective layer 1 18 covering a surface of the main body 105, and an access portion 107 connected to the main body 105. The protective layer 118 is made of a second material (herein after“non-dissolvable material”) which is anti-dissolvable or non-dissolvable to the production fluid.

The shifting sleeve 109 may be made integral to seat 108 to form a single element, or formed as a separate element and coupled to the seat 108, to form the seat assembly 106. In embodiments where the shifting sleeve is integral to the seat, the shifting sleeve and the seat move together relative to the housing, to perform dissolution operation. In embodiments where the shifting sleeve is a separate element coupled to the seat, the shifting element and the seat may move sequentially one after another, relative to the housing, to perform dissolution operation. As such, dissolution operation of the embodiments disclosed in the context and drawings with respect to an integral seat assembly is applicable in a similar manner to embodiments with separately formed shifting sleeves and seats coupled together.

Downhole tool 100 includes a sealing member, e g. a pair of O-rings 120 disposed in the housing 140. When the seat assembly 106 is at a first position fil l which is before a production fluid is injected into the downhole tool 100 for fracturing operation, shown in Figs. 1A and 2A, the O-rings 120 are positioned at both ends of the access portion 107, to isolate the access portion 107 from fluid communication with the bore 132.

Upon start of the fracturing operation, an obstructing member such as a ball 116 is dropped into the bore 132 and caught by the seat assembly 106, to close a center hole 103 of the seat assembly 106. Fracturing fluid is then injected into the bore 132 of the housing 140. Blocked by the ball 116 and seat assembly 106, the pressure of the fracturing fluid builds up to exert a force against the seat assembly 106. When the force is greater than the yield strength of the shear screw 112, the shear screw 112 is shorn off, to allow- the seat assembly 106 to move toward a second position PI 2, as shown in Figs. IB and 2B.

During the buil ding up process of the fracturing fluid in the bore 132 and before the shear screw 1 12 is shorn off, the protective layer 118 and the O-rings 120 positioned at both ends of the access portion 107 to isolate the main body 105 of the seat assembly from the surrounding environment, to prevent the seat assembly 106 from being dissolved.

Movement of seat assembly 106 toward the second position P12 opens the flow port 110, through w hich the fracturing fluid passes to perform fracturing operation on the rock formation surrounding the downhole tool 100. Meanwhile, the access portion 107 follows the movement of the seat assembly 106 and is moved away from the O-rings 120 by which, the O-rings 120 are opened from the access portion 107, hence the access potion 107 becomes exposed and in fluid communication with the bore 132, as shown in Fig. 2B. Fracturing fluid in the bore 132, together with solvents therein, can therefore reach the access portion 107 and start to dissolve the main body 105 of the seat assembly 106 through the access portion 107. As such, dissolution of the seat assembly 106 can be controlled to only start after the ball is landed on seat, and the fracturing fluid is pressured up to shift the sleeve assembly 106 to open the access portion 107.

A second embodiment of a downhole tool 300 is disclosed herein in conjunction with Figs. 3 A, 3B, 4A and 4B. Similar operation as the downhole tool 100 according to the first embodiment is applied to the present embodiment. Downhole tool 300 comprises a seat assembly 306 having a main body 305 made of dissolvable material, an access portion such as a thin flange or stopper 307 connected to the main body 305 and extending outwardly from the main body 305 toward housing 340 of the downhole tool 300, and a protective layer 318 made of anti-dissolvable or non-dissolvable material and covers the whole surface of the main body 305 and the stopper 307.

When the seat assembly 306 is at a first position, as shown in Figs. 3A and 4A, stopper 307 is engaged to housing 340 while a shear screw 312 is engaged to seat assembly 306 and housing 340, to fix the seat assembly 306 to the housing 340.

Upon start of fracturing operation, a ball 316 is dropped into the bore 332 and caught by the seat assembly 306, to close a center hole 303 of the seat assembly 306. Fracturing fluid is then injected into the bore 332 of the housing 340. Blocked by the ball 316 and seat assembly 306, the pressure of the fracturing fluid builds up to exert a force against the seat assembly 306. When the force is greater than the yield strength of the shear screw 312, the shear screw 312 is shorn off, to allow the seat assembly 306 to move toward a second position, as shown in Figs. 3B and 4B.

Movement of seat assembly 306 toward the second position opens the flow port 310, through which the fracturing fluid passes to perform fracturing operation on the rock formation surrounding the downhole tool 300. Meanwhile, the stopper 307 is shorn off or broken by the housing 340, by which, the protective layer 318 on the stopper 307 is removed and the stopper 307 becomes exposed and in fluid communication with the bore 332. Fracturing fluid in the bore 332 can therefore reach the stopper 307 and start to dissolve the main body 305 of the seat assembly 306 through the stopper 307. As such, dissolution of the seat assembly 306 can be controlled to only start after the ball is landed on seat, and the fracturing fluid is pressured up to shift the seat assembly 306 to break the stopper 307.

A downhole tool 500 according to a third embodiment is disclosed herein in conjunction with Figs. 5 A, 5B, 6A and 6B. Similar operation as the downhole tool 100 according to the first embodiment is applied to the present embodiment. Downhole tool 500 comprises a seat assembly 506 having a main body 505 made of dissolvable material, an access portion such as a recess 507 formed in the main body 505, and a protective layer 518 made of anti-dissolvable or non-dissolvable material and covers the whole surface of the main body 505 and the recess 507.

Housing 540 has a projection or barb 528 formed thereon and extends toward the seat assembly 506. When the seat assembly 506 is at a first position, as shown in Figs. 5A and 6A, projection or barb 528 is received in the recess 507, while a shear screw 512 is engaged to seat assembly 506 and housing 540, to fix the seat assembly 506 to the housing 540.

Upon start of fracturing operation, a ball 516 is dropped into the bore 532 and caught by the seat assembly 506, to close a center hole 503 of the seat assembly 506. Fracturing fluid is then injected into the bore 532 of the housing 540. Blocked by the ball 516 and the seat assembly 506, the pressure of the fracturing fluid builds up to exert a force against the seat assembly 506. When the force is greater than the yield strength of the shear screw 512, the shear screw 512 is shorn off, to allow the seat assembly 506 to move toward a second position, as shown in Figs. 5B and 6B.

Movement of seat assembly 506 tow'ard the second position opens the flow port 510, through which the fracturing fluid passes to perfor fracturing operation on the rock formation surrounding the downhole tool 500. Meanwhile, the protection or barb 528 scraps over the recess 507 to remove the protective layer 518 coated on the recess 507, by which, the recess 507 becomes exposed and in fluid communication with the bore 532. Fracturing fluid in the bore 532 can therefore reach the recess 507 and start to dissolve the main body 505 of the seat assembly 506 through the recess 507. As such, dissolution of the seat assembly 506 can be controlled to only start after the ball is landed on seat, and the fracturing fluid is pressured up to shift the seat assembly 506 to remove the protective layer 518 coated on the recess 507.

A downhole tool 700 according to a fourth embodiment is disclosed herein in conjunction with Figs. 7 A to 7D. Similar operation as the downhole tool 100 according to the first embodiment is applied to the present embodiment. Downhole tool 700 comprises a seat assembly 706 having a main body 705 made of dissolvable material, an access portion in the form of a pressure-activatable element such as a cavity 707 coupled to or formed in the main body 705, and a protective layer 718 made of anti-dissolvable or non-dissolvable material and covers the full surface of the main body 705 and the pressure-activatable element or cavity 707.

When the seat assembly 706 is at a first position, as shown in Figs. 7A and 7C, a shear screw 712 is engaged to seat assembly 706 and housing 740, to fix the seat assembly 706 to the housing 740.

Upon start of fracturing operation, a ball 716 is dropped into a bore 732 and caught by the seat assembly 706, to close a center hole 703 of the seat assembly 706. Fracturing fluid is then injected into the bore 732 of the housing 740. Blocked by the ball 716 and the seat assembly 706, the pressure of the fracturing fluid builds up to exert a force against the seat assembly 706. When the force is greater than the yield strength of the shear screw 712, the shear screw 712 is shorn off, to allow the seat assembly 706 to move toward a second position, as shown in Figs. 7B and 7D.

Movement of seat assembly 706 toward the second position opens flow ports 710, through which the fracturing fluid passes to perform fracturing operation on the rock formation surrounding the downhole tool 700. Meanwhile, the built up fracturing fluid breaks the protective layer 718 coated on the cavity 707, by which, the cavity 707 becomes exposed and in fluid communication with the bore 732. Fracturing fluid in the bore 732 can therefore reach the cavity 707 and start to dissolve the main body 705 of the seat assembly 706 through the cavity 707. In embodiments where a pressure-acti vatabl e element i s coupled to the main body 705, the built up fracturing fluid causes the pressure-activatable element to burst, to break or remove the protective layer 718 to expose the main body 705 for dissolution.

A downhole tool 800 according to a fifth embodiment is disclosed herein in conjunction with Figs. 8 A to 8D. Similar operation as the downhole tool 100 according to the first embodiment is applied to the present embodiment. Downhole tool 800 comprises a seat assembly 806 having a main body 805 made of dissolvable material, an access portion such as a channel 807 formed in the main body 805, and a protective layer 818 made of anti-dissolvable or non-dissolvable material and covers the whole surface of the main body 805. The channel 807 has a bare surface portion 850 uncovered by the protective layer 818. A protecting sleeve 844 is disposed in the channel 807 and slidable along the channel 807.

When the seat assembly 806 is at a first position, as shown in Figs. 8 A and 8C, channel 807 is blocked by a housing 840, the protecting sleeve 844 is positioned to cover the bare portion 850 in the channel 807, and a shear screw 812 is engaged to seat assembly 806 and housing 840, to fix the seat assembly 806 to the housing 840.

Upon start of fracturing operation, a ball 816 is dropped into the bore 832 and caught by the seat assembly 806, to close a center hole 803 of the seat assembly 806. Fracturing fluid is then injected into the bore 832 of the housing 840 at an entrance 842, along direction D81 . Blocked by the ball 816 and seat assembly 806, the pressure of the fracturing fluid builds up to exert a force against the seat assembly 806. When the force is greater than the yield strength of the shear screw 812, the shear screw 812 is shorn off, to allow the seat assembly 806 to move toward a second position, as shown in Figs. 8B and 8D.

Movement of the seat assembly 806 toward the second position opens the flow ports 810, through which the fracturing fluid passes to perform fracturing operation on the rock formation surrounding the downhole tool 800. Meanwhile, channel 807 is brought into fluid communication with one of the flow ports 810, to allow the fracturing fluid to pass through along direction D82. The fraction fluid moves the protecting sleeve 844 to expose the bare surface portion 850, by which, the bare surface portion 850 is brought into fluid communication with the bore 832. Fracturing fluid in the bore 832 can therefore reach the bare surface portion 850 and start to dissolve the main body 805 of the seat assembly 806 through the bare surface portion 850.

Dissolution of the seat assembly may he activated by a destroyer introduced into the downhole tool through an entrance of the bore of the downhole tool housing. The destroyer may be a mechanical means, chemical means, biological means and/or thermal means. For example, when it is desired to dissolve the seat assembly, a milling, grinding or drilling device or tool can be sent through the downhole to create a break in the protective layer on the dissolvable seat assembly. A breakthrough in the inner diameter of the seat assembly can be achieved when the milling, grinding or drilling device or tool runs through or at least cut off a portion of a top or bottom surface covered with the protective layer of the seat assembly. This milling, grinding or drilling device or tool can be fishing spears, downhole mills, other existing or yet to be created tools.

For the chemical activation means, a chemical fluid which can corrode or dissolve the protective layer is pumped through downhole by way of either running through the entire string, or jetted directly on the seat assemblies in a targeted manner, resulting in a breakthrough to dissolve the protective layer coated on the seat assembly, hence the dissolution process begins. Some example of the chemicals can be acids, organic solvents or any kind of chemicals that isn’t naturally present in the well bore at the concentrations required to create a breakthrough in the protective layer of the dissolvable seat assembly. Thus, the introduction of such chemicals can be used as an activation method for the dissolution process.

Biological means may be implemented by the introduction of bacterial or other organisms which are not typically present in a downhole environment. The bacterial or other organisms may digest or damage the protective layer of the dissolvable seat assembly to active the dissolution process.

Thermal activation can also be used to achieve a similar effect, where the protective layer is sensitive to heat and will crack or rapidly degrade (or in any other way allow a breakthrough) when subjected to higher temperatures, e.g. a temperature higher than the operation temperature of the fracturing fluid, to a level to cause the protective layer to crack or degrade. Heat can be introduced by jetting high temperature steam or by running a heating element downhole or by injecting heated fluid, or other methods.

Another method that will be introduced hereinafter is flow activation. Two changes of flow direction take place after the seat or sleeve is shifted when the dissolution of the seat begins. Firstly, the fracturing liquid pumped downhole will now start to flow radially outward into the downhole formation. Secondly, when the well is eventually brought into production, the production fluid will flow upwards through the sleeve. Any of these changes in flow direction can be used to activate the dissolution mechanism.

Fig. 9 is a flow chart illustrating a method 900 of operating a downhole tool according to an embodiment of the present invention. At block 910, an obstructing member such as a ball is deployed into a downhole tool to engage a seat assembly of the downhole tool. At block 920, a fracturing fluid is injected into the downhole tool to shift the seat assembly to open the flow ports of the downhole tool. At block 930, the fracturing fluid passes through the flow ports to perform fracking operation on a formation surrounding the downhole tool. At block 940, movement of the seat assembly exposes an access portion of the seat assembly to dissolution solution injected into the downhole tool. At block 950, the seat assembly is dissolved.

A downhole tool 1000 according to a sixth embodiment is disclosed herein in conjunction with Figs. 10A, 10B, 11A and I IB. Downhole tool 1000 comprises a mandrel 1002, an upper slip ring 1004 and a lower slip ring 1006 movably coupled to and surrounding the mandrel 1002. Upper and lower cones 1008, 1009 are disposed surrounding the mandrel and partially positioned between the upper and lower slip rings 1004, 1006, respectively. Any or ail members of downhole tool 1000 may be made of dissolvable material. A protective layer 1012 made of anti-dissolvable or non- dissolvable material, covers the whole surface of the downhole tool 1000.

In use, downhole tool 1000 is placed in a well bore 1090, as shown in Fig. 11A and 1 IB. The upper and lower slip rings 1004, 1006 are moved relative to the mandrel 1002 toward each other, along axial direction 1080. Caused by the cone 1008, the upper slip ring 1004 expands along radial direction 1082, and bites into the well bore 1090, to secure the downhole tool 1000 to the well bore 1090.

Movement of the upper slip ring 1004 breaks the protective layer 1012, to expose an access portion 1016 to allow a fracturing fluid injected into the downhole tool 1000 to reach the access portion 1016 and start to dissolve the dissolvable members of the downhole tool 1000 through the access portion 1016.

Solutions provided by this embodiment may be used to control dissolution of relevant parts in barrier devices such as pump-out-plug, bridge plug and fracture sleeve.

In some applications of fracturing operation, the protective coating and/or layer covered on a downhole tool or parts thereof, maybe subject to proppant (often sand) erosion: Fracturing operations involve pumping proppants at high speeds through the downhole string. The protective coating and/or layer of coating is easily eroded A seventh embodiment of a downhole tool 1200 is provided herein, in conjunction with Figs. 12A to 12D to solve the above-mentioned technical problems.

Downhole tool 1200 comprises a housing 1202 and a sleeve 1204 movably coupled to the housing 1202. The downhole tool 1200 further comprises a seat 1208. Die seat 1208 is made of dissolvable material and a protective layer 1220 made of anti- dissolvable or non-dissolvable material covers the surface of the seat 1208. An access portion of the seat 1208 is uncovered with the protective layer 1220. Downhole tool 1200 comprises a bushing 1222 made of hard materials such as nickel, ceramic or the like. Bushing 1222 is disposed in the seat 1208, and covers the front entrance of the seat 1208 to prevent the seat 1208 from erosion by the proppant injected with the fluid from the front entrance.

Upon start of the fracturing operation, an obstructing member such as a ball 1212 is dropped into a bore 1206 and caught by the seat 1208, to close a center hole of the seat 1208. Fracturing fluid is then injected into the bore 1206 of the housing 1202. Blocked by the ball 1212 and the seat 1208, the pressure of the fracturing fluid builds up to exert a force against the seat 1208 and fracturing fluid performs fracturing operation on the rock formation surrounding the downhole tool 1200 through flow' ports 1216.

During the fracturing operation, an access portion 1218 becomes exposed and in fluid communication with the bore 1206. Fracturing fluid in the bore 1206 can therefore reach the access portion 1218 and start to dissolve the seat 1208 through the access portion 1218. When the seat 1208 completely dissolves, only the bushing 1222 remains, as shown in Fig. 12C.

The bushing 1222 is deformable upon the pressure exerted thereon through the ball 1212 reaching a predetermined level. When the fracturing operation is completed, the ball 1212 is further pumped down to expand and pass through the bushing 1222, by deforming the bushing 1222, leaving a fully-opened inner hole in the seat 1208 for passing through the produced oil/gas, as can be seen in Fig. 12D.

A downhole tool according to an eighth embodiment is disclosed herein in conjunction with Figs. 13 A to 13E. The downhole tool is a non-barrier type of equipment e.g. a hanger 1300. Hanger 1300 comprises a mandrel 1302 having a bore 1312 therein, a sleeve 1306 and a slip ring 1304 movably coupled to and surrounding the mandrel 1302, and a locking member such as an array of pins 1310 coupled to the sleeve 1306 and the mandrel 1302, to secure the mandrel 1302 to the sleeve 1306.

In use, hanger 1300 is placed in an appropriate position e.g. a well bore 1390, and with the slip ring 1304 moved relative to the mandrel 1302 to expand the slip ring 1304, to fix the hanger 1300 in place, as can be seen from Fig 13A to Fig. 13C. Each pin 1310 is coated or covered with a protective layer 1314 to prevent the pin 1310 from being dissolved during the process of placing and setting the hanger 1300 in the well bore 1390.

When it is desired to release the hanger, the protective layer 1314 is scratched off or removed, by e.g. introducing into the hanger a tool or cutter or an appropriate type of liquid to which the protective layer 1314 is dissolvable. The protective layer 1314 can therefore be removed to expose the pin 1310, to allow the pin 1310 to dissolve. As the pins 1310 are eventually dissolved mandrel 1302 are unlocked from the sleeve 1306 to release the hanger 1300, as shown in Figs. 13D and 13E.

A plug member such as a ball 1400 for use in a downhole tool for fracturing operation according to a ninth embodiment is disclosed herein in conjunction with Figs. 14A and 14B.

Ball 1400 has a core 1402 and a protective layer 1404 formed on the surface of core 1402, to prevent the ball from being dissolved during the process of introducing the ball into a well bore.

A pressure-activatable device such as a rupture disk 1406 is built into the ball 1400 and underneath the protective layer 1404.

When the ball 1400 reaches a desired depth in a well bore and caught by a seat of a downhole tool (not shown), and upon a fracturing fluid injected into the downhole tool reaches a predetermined pressure level, the rupture disk 1406 bursts to break the protective layer above the rupture disk 1406, to expose the core 1402 of the ball 1400 to the fracturing fluid, to allow the ball 1400 to be dissolved.

The protective layer may be removed by mechanical means such as a milling tool, a cutting tool or a grinding tool or by abrasion. Alternatively, the protective layer can be temperature-sensitive, and be removed by controlling the surrounding temperature. In such alternative solutions, the protective layer can be removed in a controlled manner, without the presence of a rupture disk 1406.

It should be appreciated that even though the embodiments discussed above involve using an activation method to trigger the dissolution of a part within a downhole as a barrier, other applications where the dissolvable part is not a barrier and can facilitate the removal of a barrier are also applicable in this application. Further, the dissolution can allow other applications beyond removal of a barrier. For example, a loaded spring or vacuum chamber (any form of stored energy) held back by a dissolvable, protected member will trigger any of the activation methods mentioned above, and the dissolution of such member will unload the stored energy, and allow the controlled performance of an action downhole. This can be opening/closing of sleeves, ball valves etc., setting/releasing packers, or any other action Additionally, technical solutions described in the above embodiments can be applied to other tools under the broad inventive concept embodied therein. Such tools may include Pump Out Plugs, Fluid Loss Control Valve, Bridge Plugs, Balls, Darts, Dissolvable Plugs, etc. A dissolvable seat assembly with protective layer covered thereon and access portion connected to the main body of the seat assembly, according to embodiments disclosed hereinabove, may be implemented in such other tools to achieve efficient dissolution of seat assembly in a controllable manner.

In the case where the solutions are applied to a Bridge Plug, for example, there is no dissolving taken place, until the bridge plug is set, or even certain operation step is completed before rapid dissolving happens once trigger. Solutions provided by the embodiments of the present invention allows one set of materials for all well conditions and operation processes. This means cost per unit tool can be reduced, and stocking of inventory for quick lead times becomes possible.